An antenna is configured to selectively transmit/receive electromagnetic radiation at certain ranges of frequencies (bandwidths). If the antenna is sufficiently efficient at transmitting/receiving electromagnetic radiation at a particular bandwidth then that bandwidth is an operational bandwidth which may be used for telecommunication. An operational bandwidth is therefore a frequency range over which an antenna can efficiently operate. Efficient operation occurs, for example, when the antenna's return loss S11 is greater than an operational threshold such as 3 or 4 dB (these are expressed as a positive quantity because they are a loss).
A dipole antenna, for example as illustrated in FIG. 1A to FIG. 1D, typically comprises first and second conductive elements. The electrical lengths associated with the conductive elements results in certain frequencies of electromagnetic radiation becoming resonant. Typically, resonant modes may occur for standing waves at a multiple of half a wavelength (nλ/2) of the electromagnetic radiation. FIG. 1A illustrates a first resonant mode (first harmonic) λ/2, FIG. 1B illustrates a second resonant mode (second harmonic), FIG. 1C illustrates a third resonant mode (third harmonic) 3λ/2 and FIG. 1D illustrates a fourth resonant mode (fourth harmonic) 2λ. However, for a dipole antenna, even resonant modes (even harmonics) illustrated in FIGS. 1B and 1D are not operational and are suppressed because the input impedance at the antenna, at these frequencies, becomes large as the current at the feed becomes small.
FIG. 2 illustrates, in a plot of the return loss S11, the odd resonant modes of the dipole antenna, illustrated in FIGS. 1A to 1D. It will be appreciated that of all the resonant modes 51 of the dipole antenna, only the first resonant mode (first harmonic) and the third resonant mode (third harmonic) and similar odd resonant modes (odd harmonics) are operational. An operational resonant mode may, for example, be arbitrarily defined as one with an operational bandwidth. Using this definition, and referring to FIG. 2, it can be seen that there are no operational resonant modes corresponding to the even harmonics illustrated in FIGS. 1B and 1D.